Error message for missing return value with Result type is unclear

[bguiz]

The error message that I get looks like this:

src/main.rs:18:9: 18:11 error: unable to infer enough type information about `_`; type annotations required [E0282]
src/main.rs: 18         Ok(Response::with( (status::Ok, payload) ));
error: aborting due to previous error

The highlight is on the leading Ok, which is misleading. The actual fix was to either precede the statement with return, or to drop the trailing ;. It would be great if the compiler were to suggest this as well, as the current error message alone is cryptic.

[bguiz]

CC @huonw @michaelsproul

[huonw]

Code example:

fn foo() -> Result<u8, u64> {
    Ok(1);
}

fn main() {}

[bguiz]

Perhaps this should trigger [E0269] instead

[michaelsproul]

An ideal error message would be like the one generated here:

fn thing() -> u32 {
    7u32;
}

fn main() {}

test2.rs:1:1: 3:2 error: not all control paths return a value [E0269]
test2.rs:1 fn thing() -> u32 {
test2.rs:2     7u32;
test2.rs:3 }
test2.rs:2:9: 2:10 help: consider removing this semicolon:
test2.rs:2     7u32;
                   ^
error: aborting due to previous error

[steveklabnik]

Triage: here's today's error, no real change:

error[E0282]: unable to infer enough type information about `_`
 --> <anon>:2:5
  |
2 |     Ok(1);
  |     ^^ cannot infer type for `_`
  |
  = note: type annotations or generic parameter binding required

[Mark-Simulacrum]

Going to go ahead and mark this as E-needstest since it feels easy to regress on and medium-level bad if we do.

In particular, I think adding to the existing UI test src/test/ui/coercion-missing-tail-expected-type.rs is the right thing to do. The example code given here should be added to the existing UI test, the test run, and then the stderr file updated by cd-ing into src/test/ui and running the update-all-references.sh script. Given these instructions, marking as E-easy and E-mentor.

error[E0308]: mismatched types
 --> test.rs:1:29
  |
1 |   fn foo() -> Result<u8, u64> {
  |  _____________________________^
2 | |     Ok(1);
3 | | }
  | |_^ expected enum `std::result::Result`, found ()
  |
  = note: expected type `std::result::Result<u8, u64>`
             found type `()`
help: consider removing this semicolon:
 --> test.rs:2:10
  |
2 |     Ok(1);
  |          ^

error: aborting due to previous error(s)